Caveat: In order for outside contributions to the WebM project itself to be accepted into the WebM project codebase, Google must first assure themselves that the new code does not infringe any patents. Quite reasonably, Google wouldn't want some "helpful" outside contributer attempting any code sabotage, would they?

Other people/projects can of course do what they like within their own codebase, at their own risk, such as this project which includes WebM: http://www.ffmpeg.org/